This proposal describes a 5-year training program for the candidate, Catherine Hagan, D.V.M., to become an independent investigator in comparative experimental neuropathology. She is pursuing a Ph.D. in the Molecular and Cellular Biology graduate program at the University of Washington, Seattle;her dissertation work focuses on mechanisms of extracellular serotonin regulation in the brain. With this SERCA, Dr. Hagan will build on her foundation in brain serotonin biology to investigate serotonergic immunomodulation of microglial-mediated neuroinflammation with co-mentor Dr. John Neumaier, M.D., Ph.D., co-mentor Dr. Thomas Montine, M.D., Ph.D., and collaborator Dr. Thomas Moeller, Ph.D. Their combined expertise in serotonin pharmacology, neuropathology, and microglial biology is a unique feature of the rich training environment at the University of Washington. Career development activities will include specialized courses, such as the Molecular Neuropathology course at Cold Spring Harbor, and regular didactic activities such as a journal clubs focused on microglial biology, inflammation, and neuroscience. Microglia are the immune system of the brain and are critically involved in responses to injury and various diseases. These responses can be either beneficial or detrimental. Conditions leading to these dichotomous responses are poorly understood. The long term goal of the proposed research is to understand how serotonin and serotonin receptor mediated signaling between neurons and microglia contribute to pathogenesis of neuroinflammatory diseases. Aim 1 focuses on how the serotonin signal to microglia may be disrupted by inflammation. Rotating disk voltammetry will be used to study whether stimulating cerebral innate immunity in vivo alters neuronal serotonin transporter kinetics. This may be an underlying mechanism for how changes in serotonin lead to changes in microglial function. Aim 2 explores the possibility that microglia have an important functional capacity to take up serotonin and the mechanisms by which occurs. Aim 3 focuses on microglial serotonin receptor expression and function. Results will be used to test the hypothesis that impaired serotonergic neurotransmission and signaling potentiates microglial-mediated neurotoxicity. Immunomodulation of microglia has the potential to improve clinical outcomes in a variety of disorders, including neuroinflammatory disorders, neurodegenerative diseases, and chronic pain disorders. Accomplishing the aims of the proposed studies will determine if targeting serotonin signaling between neurons and microglia offers a new therapeutic approach to neuroinflammation through modulation of microglial responses to injury.